[Ardour-Users] click free editing and zero crossings?
lowen at pari.edu
Wed May 19 08:29:38 PDT 2010
On Wednesday 19 May 2010 08:49:12 am Jörn Nettingsmeier wrote:
> a) there is nothing magical about zero crossings.
Well, actually there is. If you're splicing arbitrary clips at arbitrary
signal levels, zero crossing is the only rough point that all signals (without
DC bias) will pass through eventually. But you can just pick a point
arbitrarily close to zero to make it work. And as Paul so rightly points out,
true 'zeros' in a signal are rather rare; you're looking for adjacent positive
and negative samples to see the zero crossing. But that makes finding zero
quite a bit easier than finding any other chosen value; just look for the sign
change instead of comparing if two adjacent samples bracket the chosen value.
So, yes, zero crossing has some magic to it; just not the magic one might
> b) you might as well edit at any <arbitrary value> crossing. what helps
> preventing clicks here is the fact that both sides of the splice are at
> roughly the same value. what helps even more is for both splice sides to
> cross <arbitrary value> in the same direction (unlike in my example,
> which was intentionally nasty in that regard).
Ideally, the signal will cross zero and there won't be audible discontinuities
in the first and second order derivatives of the signal waveform across the
> c) splices at similar values (or zero crossings, for that matter) are
> not a 100% guarantee the edit will be clean.
So true; which is one reason Audacity and many other destructive editors don't
do a sharp cut anyway; there is a very small crossfade involved, at least with
Audacity and Audition. Haven't checked Wave Editor or Cubase yet.
But good editing is good editing, and proper crossfades are essential in any
case. I'm working on a bluegrass track right now where I'm splicing banjo and
guitar breaks, and, since the artist doesn't use a click track (says it ruins
her groove (she says 'timing' but she really means groove); and she's right,
even in bluegrass, one of the more rhythmically structured genres, the
'groove' involves the ever so slight 'swaying' of the rhythm; and her timing
is very good indeed, to where the tightening of timing by using a click track
would make almost no audible difference), I have to make adjustments....
So I'm beat-matching the splicing; when done I'll re-lay the bass track rather
than edit that: since I'm the bassist, I can do that.... :-) The track is
quite fast; she did several takes of the break, and she missed a few notes on
pretty much every take on the guitar part (her banjo part is solid, but
there's that groove thing again, and when splicing in from the guitar, to get
an exact beat match (de rigeur for bluegrass is the exact timing of the
different instruments relative to each other), the banjo has to come with it),
and I'm putting together a 'best of' montage. Which is quite fun to do,
because I'm having to grok her groove and not only beat match but groove match
(first order derivative of the rhythm.....). Yes, I'm having a blast doing it.
Helps my bass playing immensely, too.
Clicks are not the only issues when editing, especially when editing pad-type
parts (fiddle, dobro, or harmonica in bluegrass) or electric bass. If the
crossfade spans more than a few cycles, you really want the actual cycles
during the crossfade to phase match; unless you are looking for a phasor effect
at the splice, that is. Zooming way in helps this; carefully picking the
splice points helps more. Percussive parts, like mandolin and guitar, aren't
as picky. Splicing banjo is the most fun, thanks to the percussive attack but
paddish sustain due to the resonator. Splicing in the resonator tail is
easiest, you just have to zoom way in and match cycles across the crossfade.
A crossfade that spans the attack but is phase matched in the resonator tail
typically makes the attack sound mushy, since virtually no two banjo notes
have phase matching between the attack and the resonator tail (most tails lead
in to the next attack anyway, and really rapid-fire banjo playing never really
gets out of any single resonator tail.......)
The easiest places I've found to splice vocals is during breaths; inhales
aren't super picky, and the crossfades aren't critical.
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